Identification of capacitation inducers customized to sperm retrieved from inbred mouse epididymis

Acquisition of sperm capacitation post-ejaculation into the female reproductive tract is an essential step in the fertilization process. Accordingly, during in vitro fertilization, successful fertilization requires the induction of capacitation in epididymis-retrieved sperm. To date, many candidate substances have been considered as capacitation inducers; however, there are no reports on the efficiency of inducing sperm capacitation among the diverse inducers. Therefore, we attempted to determine the inducer with the best capacitation in inbred mouse sperm by comparing the capacitation performance of a variety of inducers and the percentage of in vitro fertilization-generated zygotes. Calcium chloride, progesterone, bovine serum albumin (BSA), heparin, and lysophosphatidylcholine (Lyso-PC) were used as candidate capacitation inducers. Optimized concentrations of each inducer (2.7 mM calcium, 15 μM progesterone, 0.3% (w/v) BSA, 50 mM heparin, and 100 μM Lyso-PC) were determined based on the ratio of sperm showing an acrosome reaction using Coomassie G-250 blue staining. Calcium at 2.7 mM showed the strongest capacitation induction compared to the other inducers. In vitro fertilization was performed using sperm incubated in each inducer and the ratio of fertilized oocytes was determined. Calcium at 2.7 mM and 0.3% (w/v) BSA showed the highest fertilization rates compared to 15 μM progesterone, 50 mM heparin, and 100 μM Lyso-PC. From these results, we found that 2.7 mM calcium and 0.3% (w/v) BSA were the most effective sperm capacitation inducers of mouse sperm for in vitro fertilization.